The invention concerns a device employed to transmit seismic vibrations in order to obtain geophysical data on buried layers.
More particularly, the invention concerns such a device intended to be mounted on a mobile platform, such as a mobile vehicle of the truck and/or buggy type.
The active part of such a device is a vibration assembly with as its function to apply, to the ground, a variable force (of the sinusoidal type for example) of predetermined nominal value.
To this end, and referring to FIG. 1, the vibration assembly device 200 known from previous designs includes a vibrator part and a support plate 210 for coupling to the ground, with the support plate 210 used to transmit the vibrations generated by the vibrator through the ground. The vibrator part includes a reaction mass 270 and a drive piston 260, the piston 260 being used, when it is operated by a servo-valve, to guide the reaction mass 270 in a relative movement with respect to the support plate 210. The vibrator part is thus used to generate a seismic vibration wave and to transmit it into the ground.
In order to hold the vibrator part and the support plate 210 flat against the ground, a ground-hugging assembly is also included in the device 200 which is known from previous designs. This ground-hugging assembly is therefore fixed to the vibration assembly, and mounted to slide on the chassis 100 of the mobile platform serving as a firstly to place the vibration assembly on the ground and secondly to transfer the weight of the mobile platform onto the support plate 210, which thus applies a static pre-load in a direction along an axis 1. This axis 1 is parallel to the slide on the chassis 100, and is generally chosen to be perpendicular to the lower surface of the support plate 210, which is considered to be parallel to the general plane of the ground.
However, the ground on which such a device 200 is placed is never uniformly horizontal. Thus, the general plane of the ground may not be perpendicular to axis 1 and can therefore generate shear stresses in the device 200, in particular at the level of the vibration assembly coupled to the ground by means of the support plate 210. These shear stresses then tend to reduce the quality and the reliability of the device 200 if no compensation members are provided.
In order to attempt to get around these problems, referring to FIG. 2, the devices 200 of previous design include compensation members consisting of mechanical links between the vibration assembly and the ground-hugging assembly. These compensation members thus compensate for the perpendicularity anomalies of the ground in relation to the axis 1, by allowing an axial deviation 2 of the piston 260 in relation to the axis 1 of the ground hugging direction, of up to a solid angle a, and thus allowing the ground to be hugged in a consistent manner by the vibration assembly (with a maximum of contact between the vibration assembly and the ground), and thereby preserving sufficient pressure of the mobile platform on the vibration assembly, and minimising the shear stresses in the device 200.
These compensation members can also be used to isolate the ground hugging device, as well as the chassis of the mobile vehicle, from the vibrations emitted by the vibration device.
Typically, the compensation members located between the lower part of the vibration assembly and the ground-hugging assembly are composed of, or include, air cushions 252, thus allowing, pneumatically and asymmetrically in relation to the axis 1, an inclination of the vibration assembly in relation to the ground-hugging assembly, thus compensating for one part of the verticality error of the ground in relation to the axis 1 of the ground hugging direction.
The compensation members located between the upper part of the vibration assembly and the ground-hugging assembly are generally composed of air cushions 251, to compensate for part of the verticality error of the ground in relation to the axis 1 of the ground hugging direction, and of sliding skids 259 allowing a sliding motion of the vibration assembly on the ground-hugging assembly with the minimum of friction, thus compensating for at least one part of the ground perpendicularity error in relation to the axis 1 of the ground hugging direction.
Other compensation members for horizontal and/or vertical displacements can be provided, such as rubber horizontal and/or vertical skids and/or reaction bars positioned between the vibration assembly and the ground-hugging assembly.
In addition, the transfer of the weight from the mobile platform 100 to the vibration assembly via the ground-hugging assembly is accomplished in particular from above. In fact, forces transit via a lower distribution/synchronisation frame 230 (resting on the bottom cushions 252), via the sliding members 290 of the vibration assembly, and finally, via an upper distribution/synchronisation frame 248 which bears upon the sliding skids 259 by means of an intermediate structure 247. This upper distribution/synchronisation frame 248 is particularly costly and constitutes a complex element of the device 200 in relation to the other elements.
In addition, this upper distribution/synchronisation frame 248, the intermediate structure 247, and the sliding skids 259 are so many elements of the device 200 constituting a cumbersomeness, which hinders a good maintenance of the device 200.